feat(core) update Personality on turn (#9644)

## Summary
Support updating Personality mid-Thread via UserTurn/OverwriteTurn. This
is explicitly unused by the clients so far, to simplify PRs - app-server
and tui implementations will be follow-ups.

## Testing
- [x] added integration tests

codex-rs/core/tests/suite/model_overrides.rs

@@ -41,6 +41,7 @@ async fn override_turn_context_does_not_persist_when_config_exists() {
             effort: Some(Some(ReasoningEffort::High)),
             summary: None,
             collaboration_mode: None,
+            personality: None,
         })
         .await
         .expect("submit override");
@@ -84,6 +85,7 @@ async fn override_turn_context_does_not_create_config_file() {
             effort: Some(Some(ReasoningEffort::Medium)),
             summary: None,
             collaboration_mode: None,
+            personality: None,
         })
         .await
         .expect("submit override");